The use of transparent plastics for the replacement of glass windows and lenses has steadily increased in many applications from automotive (windows, headlamps and taillights) to ophthalmic lenses for vision correction and safety shielding. Transparent plastics such as polycarbonate, polymethylmethacrylate and polyurethane are lighter and more shatter resistant than glass and offer excellent transparency and low haze. However, these plastics are all much softer than glass and as such are very easily scratched under normal actions such as cleaning, wiping off dust and normal handling while in use. Over time, scratches on the surface will obscure the user's vision, eliminating the benefit of lower weight lenses.
A great deal of development has gone in to the formulation of abrasion and scratch resistant coatings for plastics over the last 20 years. For example U.S. Pat. No. 4,547,397 teaches a thermally cured sol-gel type coating for polycarbonate lenses, while U.S. Pat. No. 4,486,504 teaches an ultraviolet radiation curable silicone coating composition. The inclusion of silica colloidal nanoparticles with crosslinking alkoxysilanes is described in U.S. Pat. No. 4,348,462 as well as numerous more recent patents.
In environments where the humidity is high, or extreme temperature changes are frequent, the formation of fog on transparent surfaces is a serious issue. For example, the inside of a safety face shield such as that worn by emergency or military personnel becomes rapidly fogged by the wearer's breath during periods of physical exertion, and even in less extreme environments such as playing tennis or skiing. Vision is decreased, posing a danger to the person wearing the goggles or glasses. Fog is caused by the condensation of moisture on the surface of the lens. The moisture condenses as small beads, diffracting light and giving the appearance of haze.
Strategies to eliminate the formation of fog on lenses include coatings that work by either absorbing the moisture into the polymer matrix, or causing it to sheet out, so that the individual droplets cannot be seen. Chemistries for antifog coatings mirror those developed for abrasion resistant coatings. These range from thermally cured siloxane based or sol-gel chemistry as taught in U.S. Pat. No. 5,804,612, 2K thermally cured polyurethane coatings, as set forth in U.S. Pat. No. 5,877,254, and US applications 2004/0137155, 2007/0077399, 2008/0118658, and 2008/0207797, and radiation cured coatings as taught in U.S. Pat. No. 5,958,598, U.S. Pat. No. 5,578,378, and US application 2009/0017306. Anti-fog coatings in the prior art are not very abrasion resistant. One method for achieving a hydrophilic coating is through the use of high concentrations, for example in excess of 30-50 wt %, of hydrophilic monomers, such as polyoxyethylene di(meth)acrylates, or addition of surfactants, such as anionic or cationic surfactants, or nonionic surfactants. The pitfall of hydrophilic monomers is that in order to achieve high surface energies, the monomer must be added in very high concentrations, and they become evenly distributed throughout the coating matrix rather than concentrated at the coating surface. These monomers detrimentally affect properties such as hardness or abrasion resistance and in some cases they also do not promote adhesion with the substrate. When surfactants are used the opposite problem is observed, they concentrate at the high energy surface provided by the substrate, migrating away from the air/coating interface during curing where they are needed for permanent anti-fog effects.
In summary, there is a need for an abrasion resistant coating that has good adhesion to substrates and concentrates hydrophilic components at the air/coating interface while maintaining the high optical transparency and clarity of the substrate. Because of the natural flexibility and softness of many hydrophilic monomers, such as polyoxyethylene acrylates and methacrylates, inclusion of these monomers at high concentration degrades the mechanical durability of the coating. Because high surface tension additives are driven towards the interior of the coating (the coating/substrate interface) to minimize the total energy of the system, the inclusion of high concentrations of surfactants in a coating leads to poor adhesion without significant anti-fog effects.